feat: add benchmark for package.json deserialization

[Boshen]

Summary

• Added comprehensive benchmark suite for PackageJson::parse() to measure deserialization performance
• Made PackageJson::parse() public API to enable benchmarking from outside the crate
• Benchmarks cover various package.json complexity levels from simple to complex real-world files

Benchmark Results

The new benchmarks test different package.json sizes and parsing scenarios:

Benchmark	Time	Description
Small	~323ns	Basic name/version fields only
Medium	~1.4µs	Includes main, exports, imports, browser fields
Large	~3.5µs	Complex nested exports/imports with dependencies
Real Complex	~2.3µs	Actual package.json from fixtures
Batch (4 files)	~7.7µs	Sequential parsing of 4 different files
Parallel Batch	~27.5µs	Parallel parsing using rayon

Motivation

Package.json parsing is a critical operation in the resolver that happens frequently. Having benchmarks helps:

• Track performance regressions
• Optimize the parsing logic
• Understand the performance characteristics of different package.json structures

Test Plan

• Benchmarks compile and run successfully
• cargo bench package_json_deserialization produces consistent results
• No breaking changes to existing code (only made one internal function public)

🤖 Generated with Claude Code

[graphite-app]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.89%. Comparing base (79d5752) to head (2e31d85).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #698      +/-   ##
==========================================
- Coverage   94.90%   94.89%   -0.01%     
==========================================
  Files          12       12              
  Lines        2886     2882       -4     
==========================================
- Hits         2739     2735       -4     
  Misses        147      147              

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[codspeed-hq]

CodSpeed Performance Report

Merging #698 will not alter performance

_{Comparing feat/package-json-benchmark (2e31d85) with main (79d5752)}

Summary

✅ 3 untouched
🆕 6 new

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
🆕	batch_4_files	N/A	78.4 µs	N/A
🆕	complex_real	N/A	35.4 µs	N/A
🆕	large	N/A	46.6 µs	N/A
🆕	medium	N/A	25 µs	N/A
🆕	parallel_batch_4_files	N/A	82.7 µs	N/A
🆕	small	N/A	10.6 µs	N/A